1. Field of the Invention
This invention relates to a stereo processing system, and more particularly to a stereo processing system for reproducing a spatial sound field or constituting a three-dimensional auditory perspective.
2. Description of the Prior Art
Recently, in the field of an audio system, sound processing techniques for giving a spatially expanding or spreading sensation to listeners have been achieved by a stereo processing system having a surround-sound processor.
The most common stereo processing systems known heretofore have a delay circuit. These stereo processing systems are adapted to reproduce indirect sounds reflected by walls or ceilings of an auditorium. More particularly, the conventional systems have a delay circuit for producing sounds having some delay with respect to original sounds, so that the delayed sounds may be reproduced together with the original sounds through loudspeakers.
Although these conventional stereo processing systems can impart some sound spreading sensation to listeners by the provision of the delay circuits, the sensation these system can provide is based on mere "spreading" or "expanding" effect and the systems can not successfully provide a real auditory perspective or a spatial sound field.
There has been another stereo processing system known as a Dolby stereo system ("Dolby" is a registered trademark owned by Dolby Laboratories, Great Britain), which consists essentially of two-channel stereo amplifiers and two front and two rear (or three front and one rear), four in total, loudspeakers. According to the Dolby system, sounds which have been subjected to phase-shifting by 180.degree. are recorded and only the sounds having a 180.degree. phase difference are detected to be reproduced, in a monophonic way, through the rear loudspeaker or loudspeakers.
This Dolby stereo system can reproduce a spatial sound field to some extent, but this has a disadvantage that the back and forth movement of the sounds are too rapid or that sound expanding or spreading effect in the sideward direction is not satisfactory. Or, more essentially, this system is completely of no use to the media which have been encoded by a system other than the Dolby system.